How can I import BI materials from another blender file with python?
There is some archaic way you could do this with bpy.ops.wm.link_append() but a better way is to work with the api available via bpy.types.BlendDataLibraries.

import bpy

# using a relative path
path = "//file_name.blend"

with bpy.data.libraries.load(path) as (data_from, data_to):
    data_to.materials = data_from.materials


As a sanity check or for logging purposes, you can loop through what was appended and see if everything is there as it should be

for mat in data_to.materials:
    if mat is not None:
        print(mat.name)


As it relates to many files, it should be simple enough to extend the script to also loop through a list of filenames to append materials from. A final note, the materials appended have no users so make sure to stick them somewhere before closing your file.